Water Quickfinder

What is a USDW?

USDWs are defined broadly to include all fresh water aquifers unless
they have been specifically exempted from protection. A USDW may be in
current use as a source of drinking water, but that is not necessary.
A USDW is simply any aquifer which contains fewer than 10,000 mg/l total
dissolved solids and is currently being used as a drinking water source
or which is of sufficient volume and adequate quality to be a future source
for a public water system (25 or more connections).

What is Underground Injection?

Underground injection is the practice of placing fluids underground,
in porous formations of rocks, through wells or other similar conveyance
systems. While rocks such as sandstone and limestone appear to be solid,
they can contain significant voids or pores that allow water and other
fluids to fill and move through them. Man-made or produced fluids (liquids,
gases or slurries) can move into the pores of rocks by the use of pumps
or by gravity. The fluids may be water, wastewater or water mixed with
chemicals. Engineering can predict the capacity of rocks to contain fluids
and the technology to do so safely.

What Is an Injection Well?

The UIC Program defines an injection well as a bored, drilled, or driven
shaft whose depth is greater than the largest surface dimension; or, a
dug hole whose depth is greater than the largest surface dimension; or,
an improved sinkhole; or, a subsurface fluid distribution system. This
definition covers a wide variety of injection practices that range from
technically sophisticated wells which pump fluids into isolated formation
up to two miles below the Earth's surface, to on-site drainage systems,
such as septic systems and storm water wells, that discharge fluids a
few feet underground.

How Does the UIC Program Regulate the Wide
Variety of Underground Injection Activities?

The EPA classifies underground injection activities into five classes
for regulatory control purposes.
An injection well's classification is determined by its waste stream constituents
and its depth of injection relative to the depth of any USDWs near by.

Class
I wells emplace hazardous and nonhazardous fluids (industrial
and municipal wastes) into isolated formations beneath the lowermost
USDW. Class I wells that inject hazardous waste, are the most highly
regulated well class. They are technically sophisticated and are further
regulated under the Resource Conservation and Recovery Act (RCRA).

Class II
wells are oil and gas related wells. They may be used for disposal
of oil and gas wastes, enhanced recovery, or hydrocarbon storage.

Class
III wells are wells that inject super-heated steam, water, or
other fluids into formations in order to extract minerals such as
salt, sulfur, and uranium.

Class
IV wells inject hazardous or radioactive wastes into or above
underground sources of drinking water (USDW). These wells are banned
unless authorized under other Statutes [RCRA, Comprehensive Environmental
Response, Compensation and Liability Act (CERCLA)] for purposes of
ground water remediation.

Class V
wells includes all underground injection wells that are not
included in the other classes. This "catch all" well class
includes technically advanced deep wastewater disposal systems and
shallow "low-tech" wells, such as septic systems and cesspools.
Generally, they are shallow, on-site disposal systems, such as floor
and sink drains which discharge directly into the subsurface via
dry wells, leach fields, and similar types of drainage systems.

Exclusions: Injection practices which are not covered by the UIC
Program include individual residential waste disposal systems that
inject ONLY sanitary waste and commercial waste disposal systems
that serve fewer than 20 persons per day and that inject ONLY sanitary
waste.